In many solenoid-operated antilock braking systems, normally open fluid control valves (sometimes referred to as poppet valves) are connected in hydraulic lines coupling a hydraulic boost unit to each brake. The boost unit develops hydraulic pressure based on brake pedal pressure exerted by the driver, and a controller monitors wheel speeds to sense wheel lock-up. When an incipient lock-up condition is detected, the controller momentarily closes one or more of the valves to hold or reduce the respective brake pressure, and then quickly opens and closes the valves to progressively re-apply the boost pressure in a series of steps. The valves are cycled in this manner to maximize the hydraulic pressure applied to each brake while preventing lock-up.
The fluid control valves are typically mechanized with large diameter, on/off solenoid valves in order to satisfy the maximum flow requirements. In the re-apply phase of the control, the valves are quickly pulsed on and off to limit fluid flow. However, the pulsing of the valve repetitively accelerates and decelerates the fluid, producing hydraulic shock waves that can resonantly excite the brake lines and other elements of the system. This can unduly stress the system, and in severe cases, can result in audible noise sometimes referred to as "hammering".
One solution to the above-described problem is to replace the solenoid valves with linear control valves that can accurately position a valve pintle relative to its seat without repeated opening and closing of the valve. However, such valves cost considerably more than a simple poppet valve, and contribute to a system that is cost prohibitive.